The present invention relates to organic aminium counterions that are useful to prevent polysulfides from precipitating out of aqueous solutions, and to processes for using these counterions to produce sulfides, bisulfides, and/or polysulfides either to scavenge elemental sulfur or to prevent polysulfides from precipitating out of polysulfide solutions. The invention also relates to polysulfide solutions comprising these counterions, and to aqueous streams containing elemental sulfur complexed with these counterions. Most preferred streams contain cyanide ions before treatment, and comprise thiocyanate ions after treatment.
Many refinery streams contain hydrogen sulfide and other sulfur-containing compounds. Many processes are aimed at removing sulfur components. However, sometimes the presence of sulfur in such refinery streams actually helps to protect the iron surfaces of the equipment used to handle this stream. Ammonium bisulfide reacts with iron at the surface of such equipment, forming an iron sulfide film. Once formed, the iron sulfide film isolates the iron surface from corrosive substances, thus preventing further corrosion.
If cyanide ion is present in the refinery stream, this delicate balance is disrupted. The cyanide ion reacts with iron ions which are freed from the FeS at equilibrium to form the very stable hexacyanoferrate (II) complex anion, which is water soluble. More and more iron ions are freed from the FeS as the system strives for equilibrium. These complexed iron ions are essentially removed as a factor in reaching equilibrium, and eventually all of the iron in the FeS film is released, exposing the fresh metal surface to corrosion. The problem is particularly acute in the sour water from fluid catalytic cracking (FCC) units.
Polysulfides, such as sodium and ammonium polysulfide, are known to control cyanide ions in waste water streams. Polysulfides convert the cyanide ions into noncorrosive, biodegradable thiocyanate ions. Thiocyanate ions do not react with FeS and are very water soluble, rendering them readily removable by water washing.
One problem with known polysulfides, such as ammonium polysulfide, is that they decompose under typical storage conditions, resulting in precipitation of free sulfur. As a result, the amount of active ingredientxe2x80x94sulfurxe2x80x94is greatly reduced, or even negligible. The precipitated sulfur also can foul or clog valves, pumps and lines, leading to expensive corrosion repair problems.
Stabilized polysulfide solutions with good cyanide scavenging capabilities are needed.
The present invention provides a composition comprising elemental sulfur complexed with organic aminium counterions, said composition having the following general formula:
HOxe2x80x94(CH2)nxe2x80x94N+R1R2R3 Sx=R3R2R1N+xe2x80x94(CH2)nxe2x80x94OH
wherein n is from about 1 to about 6; x is from about 2 to about 10; and R1, R2, and R3 independently are selected from the group consisting of hydrogen, hydroxyl groups, up to two aryl groups, alkyl groups having up to about 6 carbon atoms, and groups having the following general structure
R4xe2x80x94Yxe2x80x94R5xe2x80x94Z
wherein R4 is selected from the group consisting of hydrogen, a hydroxyl group, an aryl group, and alkylene groups having up to about 6 carbon atoms; when R4 is selected from the group consisting of hydrogen, a hydroxyl group, and an aryl group, R5 and Z are nothing; when R4 is an alkylene group having up to about 6 carbon atoms, R5 is selected from the group consisting of hydrogen, a hydroxyl group, an aryl group, and an alkylene group having up to about 6 carbon atoms; when R4 is selected from the group consisting of hydrogen, a hydroxyl group, and an aryl group, Y is nothing; when R4 is an alkylene group having up to about 6 carbon atoms, Y is selected from the group consisting of a hydrogen atom, a hydroxyl group, an xe2x80x94NH group, a sulfur atom, an alkylene group having up to about 6 carbon atoms, provided that, when Y is a hydrogen atom or a hydroxyl group, R5 and Z are nothing; when R5 is selected from the group consisting of hydrogen, a hydroxyl group, and an aryl group, Z is nothing; when R5 is alkylene group having up to about 6 carbon atoms, Z is selected from the group consisting of a hydrogen atom, a hydroxyl group, an NH2 group, and a thiol group.
Commercially available forms of polysulfides generally are aqueous ammonium polysulfide solutions having concentrations of polysulfides in the range of from about 30 wt % up to about 70 wt %. These solutions contain polysulfide ions whichxe2x80x94when the solution is added to a cyanide-containing streamxe2x80x94react with cyanide ions in the stream to form thiocyanate ions.
The compounds of the present invention are elemental sulfur atoms complexed with organic aminium ions, which have the following general formula:
HOxe2x80x94(CH2)nxe2x80x94N+R1R2R3 Sx=R3R2R1N+xe2x80x94(CH2)nxe2x80x94OH
wherein
n is from about 1 to about 6, preferably from about 2 to about 4;
x is from about 2 to about 10; and
R1, R2, and R3 independently are selected from the group consisting of hydrogen, hydroxyl groups, up to two aryl groups, linear, branched, and cyclic alkyl groups having up to about 6 carbon atoms, and groups having the following general structure
R4xe2x80x94Yxe2x80x94R5xe2x80x94Z
xe2x80x83wherein
R4 is selected from the group consisting of hydrogen, a hydroxyl group, an aryl group, and alkylene groups having up to about 6 carbon atoms;
when R4 is selected from the group consisting of hydrogen, a hydroxyl group, and an aryl group, R5 and Z are nothing;
when R4 is an alkylene group having up to about 6 carbon atoms, R5 is selected from the group consisting of hydrogen, a hydroxyl group, an aryl group, and an alkylene group having up to about 6 carbon atoms;
when R4 is selected from the group consisting of hydrogen, a hydroxyl group, and an aryl group, Y is nothing;
when R4 is an alkylene group having up to about 6 carbon atoms, Y is selected from the group consisting of a hydrogen atom, a hydroxyl group, an xe2x80x94NH group, a sulfur atom, an alkylene group having up to about 6 carbon atoms, provided that, when Y is a hydrogen atom or a hydroxyl group, R5 and Z are nothing;
when R5 is selected from the group consisting of hydrogen, a hydroxyl group, and an aryl group, Z is nothing;
when R5 is alkylene group having up to about 6 carbon atoms, Z is selected from the group consisting of a hydrogen atom, a hydroxyl group, an NH2 group, and a thiol group;
wherein two or more of R1, R2, and R3 may join together to form a cyclic or heterocyclic structure comprising up to 6 members.
In a preferred embodiment, the compound is xe2x80x9chydroxyethylaminium polysulfide,xe2x80x9d in which n is 2, R1, R2, and R3 are hydrogen, and x is from about 2 to about 10, most preferably 4.
Although the foregoing gives specific examples of organic aminium ions encompassed by the present invention, the invention encompasses organic aminium ions as long as their amine precursors are water soluble and sufficiently basic to form stable alkylaminium sulfide and polysulfide salts. The hydroxyalkylaminium sulfides or polysulfides may be used to solubilize unwanted sulfur. The hydroxyalkylaminium polysulfides may be used to provide cold stability to ammonium polysulfide solutions.
Where used to stabilize a polysulfide solution, the solution contains a sufficient amount of the organic aminium ions to maintain cold stability down to temperatures at which the resulting solution will be stored. Such temperatures will differ depending upon the customer and the end use for the particular product. Hydroxyalkylaminium polysulfides are more stable at low temperatures (i.e., will remain soluble in the customer""s storage tank even when the temperature drops to about 40xc2x0 F. or less), and have a much less obnoxious odor than ammonium polysulfide solutions and the vapors may contain less of the very toxic H2S.
Polysulfides having the foregoing structure may be made using several methods. These methods will be described using hydroxyalkylaminium counterions to form stable polysulfide solutions; however, other organic aminium sulfides or polysulfides may be used to solubilize elemental sulfur, whether to form stable polysulfide solutions or to scavenge unwanted elemental sulfur, using similar procedures.
In a preferred method for stabilizing a polysulfide solution using hydroxyalkylamines, a selected amount of a hydroxyalkylamine is mixed with hydrogen sulfide gas to produce aqueous hydroxyalkylaminium sulfide and bisulfide. The amount of hydroxyalkylamine is selected by the quantity of polysulfide solution to be stabilized and the amount of cold stability desired. If a given application requires cold stability down to about 0xc2x0 F., then the amount of hydroxyalkylamine required typically will be about 5 wt. % of the polysulfide solution to be stabilized. On the other hand, if the application requires cold stability down to about xe2x88x9220xc2x0 F. to about xe2x88x9230xc2x0 F., then the amount of hydroxyalkylamine required will be from about 10 wt. % to about 15 wt. % of the polysulfide solution to be stabilized.
Once the hydroxyalkylamine is mixed with the hydrogen sulfide gas, the intermediate aqueous solution is mixed with sulfur, preferably with a stoichiometric amount of sulfur, such that the mole ratio of sulfur to aminium sulfide preferably is about 3:1. Although adducts ranging from 1:1 to 9:1 may conceivably be used under certain conditions, the primarily 3:1 adduct of hydroxyalkylaminium polysulfide is preferred. It may be desirable to then add even more of a hydroxyalkyl amine or another water soluble base, such as a hydroxide of an alkali or alkaline earth metal, such as sodium or potassium, or any other compatible base , such as tetramethyl ammonium hydroxide, in order to raise the pH of the solution to about 10 or greater, preferably greater than about 10, most preferably about 11.
In another, less efficient method, an ammonium polysulfide solution is mixed with an amount of hydroxyalkylamine sufficient to achieve cold stability. This typically requires about 5 wt. % or more, preferably about 10 wt. % or more, most preferably about 15 wt. % or more. The intermediate solution comprises a combination of (a) monoalkanolamine and ammonium polysulfide and a combination of (b) hydroxyalkylaminium polysulfide and ammonia.
Where all three of R1, R2, and R3 are alkyl groups, an aqueous ammonium polysulfide solution is provided. Again, adducts where the ratio of sulfur to aminium sulfide ranges from 1:1 to 9:1 may conceivably be used under certain conditions, but the primarily 3:1 adduct of hydroxyalkylaminium polysulfide product is preferred. An amount of hydroxyethyltrialkyl ammonium hydroxide (HOxe2x80x94CH2CH2N+R3xe2x88x92OH) is added, preferably an amount which is stoichiometric with the mols of NH4+ present. Although a stoichiometric amount is preferred, an amount which is more or less than a stoichiometric amount also will render the resulting polysulfide solution more stable than an ammonium polysulfide solution. If desired, the ammonia may be removed by sparging from the solution with an inert gas.
The invention is intended to cover the conversion of a stabilizing amount of polysulfide ions in the subject solutions to hydroxyalkylaminium polysulfide. In a preferred embodiment, the amount of polysulfide ions converted to hydroxyalkylaminium polysulfide is effective to reduce precipitation of free sulfur and/or aminium or ammonium polysulfide during storage or use at ambient temperatures for at least about 30 days when compared to ammonium polysulfide solutions comprising substantially the same amount of polysulfide which have not been stabilized according to the present invention. In a preferred embodiment, precipitation is reduced at ambient temperatures for at least about 45 days, most preferably for at least about 75 days. Crystallization and precipitation of free sulfur and the aminium polysulfide from the solution also preferably is reduced at sub-ambient temperatures of less than about xe2x88x929xc2x0 C. (15xc2x0 F.), most preferably at less than about xe2x88x9212xc2x0 C. (10xc2x0 F.).
The hydroxyalkylaminium polysulfide may be used in methods of controlling cyanide ion concentration in any aqueous stream containing such ions. An example is the sour process water system of an FCC unit such as taught in U.S. Pat. No. 4,508,683, incorporated herein by reference. In such water streams or systems, the solutions of the present invention are injected into the system at the desired location, which readily can be determined by persons of ordinary skill in the art. An xe2x80x9ceffectivexe2x80x9d amount of the solution is injected. An xe2x80x9ceffectivexe2x80x9d amount is an amount which is effective to control cyanide ion in the stream, preferably an amount sufficient to provide an amount of polysulfide ions which is approximately stoichiometric with the amount of cyanide ions.
The pH of the aqueous system being treated preferably is from about 7.5 to about 9.5, most preferably from about 8.5 to about 9. Acidic pH""s are to be avoided because they promote rapid decomposition of the polysulfides. The amount of solution added will depend upon the concentration of cyanide ions (including HCN) in the aqueous stream being treated, the concentration of the polysulfide in the solution, and the degree to which it is desired to reduce the cyanide ions in the aqueous stream.